The present invention relates to an automatic gain control circuit for radio receivers and more particularly to a circuit for correcting a discontinuous gain control function to provide a linear logarithmic gain control versus signal level function and a continuous AGC signal derived from both envelope and synchronous AM detectors.
The receiver with which the present invention is to be employed is a telemetry receiver for acquiring information from satellites and deep space probes in which high noise levels are quite common. In order to obtain a reasonably good noise figure over the most sensitive range of operation of the receiver, no gain correction is introduced by the tuner of the receiver over the lowest 40 dB range of signal levels to be received.
A problem arises from the fact that in the type of work with which such receivers are employed, it is often desired to record the information received for further processing at a later time. The AGC system described above, however, distorts the AGC curve due to the non-linearity introduced by suppression of AGC action in the tuner over approximately the first 40 dB range of signal levels. Specifically, even though the response of the system to the AGC signal, when invoked, is linear with respect to logarithmic carrier level, the fact that the AGC signal is ineffective at low carrier levels results in an overall non-linear logarithmic control function. Thus, the tuner does not provide undistorted AGC data for subsequent processing.
Another problem encountered in the receiver of the present invention results from the use of both envelope and synchronous AM detectors. The synchronous AM detector is employed to detect signals at levels as low as 20 dB below noise while the envelope detector does not operate normally below zero dB signal levels relative to noise. Thus, if the two signals were applied throughout the system, when the synchronous detector locked on the carrier, an AGC signal would be switched in indicative of a 20 dB jump in signal; i.e. receiver gain would be reduced by 20 dB. Such a rapid change in level is acceptable in some parts of the system. If it is desired to record the signal, as is the case in the environment to which such a receiver is applicable, the recording is rendered virtually useless.